My present invention relates to a common antenna for primary and secondary radar systems.
Generally, when the antenna of the secondary radar is integrated with the antenna of the primary radar, thus forming a combined primary/secondary antenna structure, this bifunctional antenna comprises a single reflector illuminated by an associated source in such a way as to be capable of radiating outgoing energy into space for the purpose of detecting a target such as an aircraft, this being called the primary radar function, and also of emitting an interrogation signal in the direction of this aircraft which is assumed to be equipped with an onboard automatic responder termed a transponder, this being called the secondary radar function.
The radiated beam conveying the interrogation signal is effective in the direction where the aircraft has been detected; however, it has been noticed that the transponder of the interrogated aircraft could be triggered by the secondary lobes of the interrogation pattern whose level is liable to be relatively high with respect to that of the principal lobe. To remedy this disadvantage, as noted in my above-identified prior patent, the single antenna here considered can be provided with supplemental radiating elements affecting the reception of the interrogation signal by the remote transponder as well as the reception of the response from the latter by the local receiver; these elements radiate in a substantially omnidirectional control pattern whose level is such as to blank the secondary lobes of the interrogation pattern.
By means of a comparison in the associated circuits of the amplitude of the pulses received from the transponder with those received from the control system, this arrangement facilitates a detection of the pulse received in response to the interrogation by the principal lobe. The means for establishing the control pattern must be such that the gain of the associated control channel is greater than that of the interrogation and reception channel in the angular zones comprising secondary lobes of the directional interrogation pattern but much smaller in the direction of the principal lobe.
According to the disclosure of my above-identified copending application and patent, the secondary radar function is performed by a row of linear radiating elements or transceivers, totally integrated in the reflector of the antenna, along a generatrix intersecting the boresight axis of the antenna by passing through the projection of its phase center onto the concave reflector surface. In that system the emission-reception source of the primary radar function radiates a wave which is polarized rectilinearly and orthogonally to the rectilinearly polarized wave emitted by the radiating elements of the secondary radar function.